JPS5810694A - Method of detecting failed fuel assembly - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to the identification of damaged fuel assemblies in fast breeder reactors.
This invention relates to a method for detecting damaged fuel assemblies by inserting and withdrawing control rods (confirmation).

The fuel assembly of a fast breeder reactor consists of a large number of fuel elements (also called fuel bins) arranged in a trumpet tube in an evenly spaced arrangement with spacers interposed between them. Fissile material, such as UO2, in the loaded fuel pellets reacts with neutrons and undergoes fission, producing a large amount of fission product fragments.

A fuel element generally comprises a cylindrical cladding tube made of, for example, stainless steel, and end plugs for closing the upper and lower ends of the cladding tube. The structure is such that the fission products produced by nuclear fission are contained within the cladding and are not released to the outside.

However, during operation of a nuclear reactor, there is a possibility that the cladding tube may break due to some unpredictable cause, and fission products that should be contained may be released outside the fuel element.

Since operating a nuclear reactor with a damaged fuel element is not desirable from a safety or operational perspective, fast breeder reactors are generally equipped with a detection device that can quickly detect damaged fuel. ing.

Conventionally, two types of detection devices are known.

One method is to sample the cover gas sealed above the coolant in the reactor vessel and measure whether the cover gas contains gaseous components of fission products. It's called the law. The other method is to install a neutron detector in the primary cooling system piping connected to the reactor vessel and detect damage to the fuel assembly by measuring continuous neutrons from fission products contained in the coolant. ,D
It is called the N method. However, in normal usage, the above two methods can confirm whether or not the fuel assemblies in the core are damaged, but they cannot identify which fuel assemblies in the core are damaged. There is a drawback that it cannot be done.

The present invention has been made in order to eliminate the above-mentioned drawbacks, and the present invention has been made to eliminate the above-mentioned drawbacks, and to replace the fuel assembly containing the damaged fuel element with the above-mentioned CG? ! i! i
: To provide a method for detecting damaged fuel assemblies that can easily and quickly identify the location of damaged fuel assemblies in the reactor core using the M/DN method.-10 (B-10) This is a method for detecting damaged fuel assemblies, which is characterized by inserting control rods in descending order of the number of control rods.

An embodiment of the present invention will be described below with reference to the drawings. Note that the present invention is not limited to this embodiment, but can be applied to other embodiments having the same meaning.

The figure is a conceptual plan view of the core of a 00 MW class fast breeder reactor with a heat output cuff viewed from above. As shown in the figure, multiple fuel assemblies containing multiple fuel elements housed in wrapper tubes are arranged hexagonally in a honeycomb shape to constitute the reactor core.

In addition, the parts B1 to B5 and C indicated by circles within the hexagonal frame in the figure
1 to 13 represent the positions of the control rods, and B to B6 represent the backup reactor shutdown rods of the control rod assembly each containing approximately 43,002 Boo loads.

During normal operation, these control rods are generally withdrawn, and at the end of the operating cycle, when fuel damage is most likely to occur, almost all of the control rods are withdrawn.

Here, it is assumed that fission products are detected by the CG method or the DN method during normal operation.

For example, after a certain period of time has elapsed after a fuel element is damaged, the release of fission products from the damaged hole becomes constant and becomes a value proportional to the production rate of fission products, that is, the output level. The signal level of the radiation detector becomes constant.

However, in this state, the correlation between the output level of each fuel assembly or the output level of the fuel element and the signal level of the radiation detector is not clear, so it is difficult to quantify which fuel element in which fuel assembly is damaged. I can't do that.

Therefore, in the present invention, in this state, the output level of each assembly is varied by manually inserting and withdrawing the control rods, and the C
By utilizing the signal level of a radiation detector using either the G method or the DN method, fuel assemblies containing damaged fuel elements are identified.

In this method, first, the control rod with a large Boo loading in the figure,
That is, B1. B2. B3. B4. 1 in the order of B5-B6
Insert and withdraw control rods one by one. Here, for the six surrounding control rods, that is, the B2 control rod, the output level after insertion for the six fuel assemblies 2 is 7 of the output level before insertion, assuming the total power of the core is constant.
It will be below 0.

Therefore, if the fuel assembly surrounding the damaged fuel element is included in the six surrounding control rods inserted, the signal level of the radiation detector of the damaged fuel detection system will be the same before the insertion, assuming the core output is constant. Compared to that, after insertion it is less than 70%. Furthermore, if any of the fuel assemblies within the outer row is damaged, the ratio changes to 70 or more and 90% or less.

In this way, the core output decreases due to control rod insertion, but the total core output before and after insertion is assumed to be constant in calculations, and the radiation detector signal levels are compared, so before comparing the signal levels. Normalized by reactor thermal power meter.

Similar normalization is performed for the control rod operations shown below, and the signal levels are compared under constant full core power.

Therefore, in the middle of the operation procedure for six control rods, if the signal level of the radiation detector after insertion of a certain control rod becomes 90% or less after the normalization of the full core power, the three control rods surrounding the control rod It is assumed that the fuel assembly in the column contains a damaged fuel element,
The following control rod operations will not be performed. Next, operate the control rods around the control rod as follows.

That is, for example, in the figure, if a signal fluctuation of less than 0 - is obtained by inserting the B2 control rod, Bs, C3°cl, C
The control rods are inserted and withdrawn in the order of 2, C12, and C7, and the signal level is normalized in the same manner as described above. In this case, if the normalized signal fluctuation due to the insertion of the B2 control rod is less than a factor of 70, if one of the two fuel assemblies adjacent to the B5 control rod contains a damaged fuel element, then
The change in signal level after normalization is 80 degrees or less. For other control rods, that is, one fuel assembly with two adjacent control rods, C3, C1, C2, C12, and C7K, it will be 9 liters or less. Since the fuel assembly adjacent to each control rod overlaps one fuel assembly adjacent to an adjacent control rod, it is possible to identify which fuel assembly contains the damaged fuel element.

In addition, the change in signal level after normalization due to the B2 control rod is 7
If the normalized signal level becomes 701 or less due to the insertion of the B5 control rod, it will contact the B5 control rod and the second and third rows counting from the B2 control rod will be inserted. It is assumed that one of the second fuel assemblies 3 contains damaged fuel. In this case, which of the four fuel assemblies 3 is the damaged paint assembly can be identified using the same method as above based on the change in the normalized signal level after insertion of C7, B2, and C3. I can do it. Similarly, after normalization using the KB2 control rod, the change in signal level is between 70% and 90%, and the signal level after normalization due to the insertion of any of C3, C1, C2, C12, or C7 is 80qb or less. In this case, it is assumed that one of the four assemblies in contact with the control rod contains a damaged fuel assembly, and the closest multiple controls around the control rod that were similarly inserted and withdrawn are It is possible to identify which of the four rods is a damaged fuel assembly from the change in signal level caused by rod insertion.

In the figure, the fuel assemblies near the outside of the core are not surrounded by control rods, so if there are damaged fuel assemblies in this area, it is difficult to identify them, but the same operation as above can be used. Yes, it will be possible to identify at most two bodies.

In addition, if the embodiments of the present invention are listed, the following procedures will be followed.

(]) Wait until the signal level of the radiation detector using the CG method or DN method becomes constant. Hereinafter, the same applies to the insertion and withdrawal operations of each control rod.

(2) Insert and withdraw one of the high BIO loading control rods.

(3) Compare the signal level of the radiation detector with the reactor output constant (normalized) when the control rod is inserted and withdrawn.

(4) Repeat the steps (2) and (3) above for other high BIO loading view control rods.

(5) During the steps (1) to (4) above, there is a change in the signal level of the radiation detector after normalizing the reactor output.When the signal level exceeds a certain value before and after inserting a control rod with a high BIO loading. , do not insert the control rod with a high BIO load after that and pull out the control rod.

(Q) In (5) above, perform the insertion and withdrawal operations of the six closest control rods in order of BIO loading in descending order of the control rods surrounding the inserted control rod, and calculate the signal level after normalizing the reactor output. compare.

(7) If the change in the signal level in (5) above is greater than a certain value, it is assumed that the fuel adjacent to the control rod in (5) is damaged, and which of the fuels (6 ) is determined by the magnitude of the change in signal level.

(8) When the signal level change in (5) above is below a certain value,
And if it is above the certain value shown in (5), then proceed with step (
The control rod closest to the control rod where the change in signal level exceeded the specified value in 6), and in (5) and (6) above.
The changes in the signal level were measured by inserting and withdrawing the three control rods that were not inserted or withdrawn in the above (5).
, The position of the damaged fuel is determined using the same judgment as in (7), also using the signal level change data obtained in (6).

As explained above, according to the method for detecting a damaged fuel assembly according to the present invention, a damaged fuel assembly can be identified or specified at an early stage by the simple method of inserting and withdrawing control rods, and the reactor operation This has the effect of improving performance, availability, and safety.

[Brief explanation of drawings]

The figure is a conceptual plan view of the reactor core viewed from above to explain an embodiment of the method for identifying damaged fuel assemblies according to the present invention. 1.2...Fuel assembly, B1-B6...Control rod with large BIO loading, C1-C
13...Control rod with a small amount of BIO loading. Application agent Patent attorney Gobe Kikuchi

Claims (2)

[Claims] (1) Insert and pull out the control rods into the reactor core,
A method for detecting a damaged fuel assembly that fixes a damaged fuel assembly, the method comprising inserting and withdrawing control rods in order of increasing boron-10 (B-10) loading. (2) A patent that is characterized in that when a change in the amount of detected radioactivity exceeds a specified value, the insertion and withdrawal operations are repeated in sequence for a plurality of control rods closest to the inserted or withdrawn control rod. A method for detecting a damaged fuel assembly according to claim 1.